The present invention relates to cement products and in particular relates to the recovery and recycling of cement kiln dust. The present invention further relates to the recovered cement kiln dust and its use in cement products.
As stated in U.S. Pat. Nos. 5,156,676 and 4,584,022 (both incorporated in their entirety by reference herein), the calcining and clinkering of cement ingredients can be accomplished by a number of ways. The typical process uses a rotary kiln, either wet or dry. The cement ingredients, i.e., the cement raw materials, are finely ground and intimately mixed to provide a substantially homogenous mixture. This homogenous mixture is added to the input or feed-end of the kiln and is commonly referred to as the raw kiln feed. The kiln is tipped downwardly at an angle such that the heat-end of the kiln is below the feed-end. The kiln has generally four operating zones including a precalcining zone, a calcining zone, a clinkering zone, and a cooling zone. Conventional fuel is combined with preheated air and injected into the kiln at the heat-end. Fuels such as natural gas, oil, or pulverized coal are conventionally used in cement manufacturing.
As the finely divided cement raw materials pass into the rotating kiln at the feed-end, the kiln feed is heated from near ambient temperature to about 1000xc2x0 F. in the precalcining zone. In this zone, the heat of the combustion gases from the calcining zone is used to raise the temperature of the raw materials. Additionally, in the kiln, chain systems or the like may be attached to the interior of the kiln and are employed to improve the efficiency of heat exchange between the gases and raw materials.
The temperature of the raw materials is increased from about 1000xc2x0 F. to about 2000xc2x0 F. as they pass through the calcining zone. This calcined material then passes into the clinkering or burning zone where the temperature is raised to about 2700xc2x0 F. It is in this zone that the primary raw materials are converted into the typical cement compounds. The cement clinkers then leave the clinkering zone where the clinkers are cooled and processed further such as by grinding.
Exhaust gases from the rotary kiln exit and the feed-end of the kiln entrain fine particles of the cement raw materials being supplied to the kiln. These fine particles are collected in high efficiency dust collectors such as fabric filters or electrostatic precipitators. These collected particles are referred to as cement kiln dust (CKD) or dust catch.
As mentioned earlier, the process may involve either a wet or dry rotary kiln. When a wet rotary kiln is used, the raw kiln feed is a slurry of the cement raw materials and water. Any water introduced into the kiln will increase fuel costs. Since volatilization of the water requires energy, the percent solids of the kiln feed slurry is kept to a maximum, using only enough to achieve a kiln feed slurry that can be pumped into the kiln at a reasonable cost. In some cases, it may be cost effective to use a specific type of chemical additive that can reduce the viscosity of the kiln feed slurry. Such an additive is commonly called a slurry thinner or dispersant and can minimize the water required while still maintaining a slurry viscosity that can be pumped cost effectively.
The cement kiln dust (CKD) that escapes from the kiln is not suitable for use in cement and so is generally disposed of in a landfill. Since CKD can represent as much as 15% by weight of the cement clinker actually produced, there are considerable economic benefits that can be gained if the CKD can somehow be reused in the cement making process. One means of doing this is to feed the CKD dust into the kiln at the heat-end of the kiln, a method commonly referred to as insulfflation. However, this process in the best case can only reuse about 50% of the CKD generated.
Accordingly, there is a need in the industry to develop methods and apparatus to more efficiently recover and use the cement kiln dust generated by cement manufacturing plants and to incorporate this cement kiln dust into finished cement clinker.
A feature of the present invention is to provide a method and apparatus for recycling cement kiln dust.
Another feature of the present invention is to provide products made from the recycled kiln dust.
Additional features of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and obtained by means of the elements and combinations particularly pointed out in the written description and appended claims.
To achieve these and other advantages, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention relates to a method for recycling cement kiln dust. The method involves heating a mixture containing at least one additive, water, and cement kiln dust for a sufficient time and at sufficient temperatures to form cement clinker. The additive contains at least an organic polyacid, a hydroxy carboxylic acid, a polycarboxylic acid, a saccharide, a carbohydrate, a salt thereof, or combinations thereof.
The present invention further relates to a method for recycling cement kiln dust involving the recovery of cement kiln dust, the blending of the cement kiln dust with water and at least one additive to form a slurry and then feeding the slurry into a kiln. The slurry is then heated for a sufficient time and at sufficient temperatures to form cement clinker. The additive is the same additive as described above.
The present invention also relates to a cement slurry comprising water, cement kiln dust, and at least one additive selected from an organic polyacid, a hydroxy carboxylic acid, a polycarboxylic acid, a saccharide, a carbohydrate, a salt thereof, or combinations thereof.
Furthermore, the present invention relates to a system for recycling cement kiln dust which includes a cement dust kiln dust collector in communication with means to transfer the kiln dust from the cement dust kiln dust collector to a pre-mixer. The pre-mixer also receives a water feed line and an additive feed line wherein the pre-mixer mixes together the cement kiln dust, the water, and additive together to form a slurry. In communication with the pre-mixer is a final mixer, such as a stirred tank-type mixer which is equipped with baffles and a mixing impeller. Connected to the final mixer is a slurry delivery pump which conveys the cement kiln dust slurry directly back to a kiln feed to be introduced into a cement kiln. The cement kiln dust recycling system can further include a variety of additional equipment and instruments such as flow meters, metering pumps, rotary valves, dilution systems, and other related equipment.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide a further explanation of the present invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the present invention and together with the description, serve to explain the principles of the present invention.